Silver halide photographic emulsions are typically prepared by combining aqueous solutions of silver salt with halide salt in the presence of a gelatino-peptizer protective colloid. Following precipitation of silver halide crystals, excess salts may be removed by a variety of techniques as disclosed in the prior art, including a coagulation process in which a protective colloid is coagulated with a coagulating agent and the resulting supernant solution is removed, as disclosed in Japanese Kokais JP-A 48-13057, 59-166939, 60-220337, 60-52844, 62-212642, 62-237993, 63-214743 and 2-272442; a noodle washing method in which an emulsion is cooled down to be set and shredded in the form of noodles to be washed as disclosed, for example, in JP-A 47-44487 and 49-46255; and ultrafiltration by use of high pressures and a semipermeable membrane, as disclosed in JP-A 57-209823, 59-43727, 62-113137, 2-172816, 2-172817 and 3-140946.
More recently, processes of applying ion-exchange or electrodialysis have been suggested to overcome various problems associated with prior emulsion desalting methods, as disclosed in, e.g., JP-A 61-219948, 62-23035, 63-40137, 63-40039 and 4-22942. FIG. 1 is a schematic illustration showing the manner of electrodialysis. In FIG. 1, a salt in a feed solution circulated through pump P.sub.1 is dissociated into a cation (e.g., Na.sup.+) and an anion (e.g., NO.sub.3.sup.-). C indicates a cation-exchange membrane which permits only cations to pass therethrough, while A indicates an anion-exchange membrane which permits only anions to pass therethrough. A cathode (-) is placed in the outside of the C-membrane and an anode (+) is placed in the outside of the A-membrane. When a direct current voltage is applied between both electrodes, cations in the solution are attracted toward the cathode, passing through the C-membrane to reach the cathode, while anions are attracted toward the anode, passing through the A-membrane to reach the anode. The cations and anions are transferred to a waste stream circulated through pump P.sub.2. Anions transferred to the anode-side and cations transferred to the cathode-side cannot pass through the A-membrane and C-membrane, respectively. Since ions which have passed through the ion-exchange membrane cannot return to the original solution, desalting is consequently accomplished.
The main commercial applications of electrodialysis are in desalting of sea water or brackish water. In these applications it is important to achieve high efficiency of deionization and minimum loss of water. As stated above, however, silver halide emulsions are desirably simultaneously dewatered (i.e., concentrated) as well as deionized during washing. Deionization is easily achieved by conventional electrodialysis, and in the course of electrodialysis ions are transferred with water molecules hydrated to the ions, thereby concentrating the emulsion to a slight degree. The amount of salt that is usually present in the emulsion (or feed stream) after precipitation, however, is typically not sufficient to achieve the desired amount of dewatering as a result of ionic hydration.
U.S. Pat. No. 5,523,201 discloses a process in which additional salt is added to a silver halide emulsion during electrodialysis to achieve a desired level of dewatering. However, the addition of extra salts to the emulsion may have detrimental effects on the stability and performance of the emulsion. Furthermore, one is severely restricted in terms of the type of salt that can be added to the emulsion. Halide salts such as sodium chloride, e.g., cannot be used because of concerns around ripening of the emulsion. High levels of divalent or higher valency salts may cause precipitation of the gelatin in the emulsion. While the use of additional nitrate salts in the feed such as sodium or potassium nitrate may minimize effects upon the emulsion, there are ecological concerns about the level of nitrate in the effluent. It would therefore be advantageous to devise an efficient process for deionizing and dewatering an emulsion by electrodialysis where one does not have to add additional salt to the feed.